Correctly sequence the events that lead to the contraction of skeletal muscle. T
ID: 3506712 • Letter: C
Question
Correctly sequence the events that lead to the contraction of skeletal muscle. These events should include the action potential in an alpha-motor neuron, neuromuscular junction, and crossbridge cycling. The first step has been given below. Follow this step with 13 additional steps (in order) that lead to muscle contraction. Feel free to add more steps if that is what you need to get to crossbridge cycling. This is all the information we were given******
1) EPSPs summate on the -motor neuron until a threshold potential is reached
2)
3)
4)
5)
6)
7)
8)
9)
10)
11)
12)
13)
14)
Explanation / Answer
Muscle contraction in skeletal muscle requires the stimulation of the muscle by an action potential from the motor neuron that connects to the muscle fiber. One motor neuron will stimulate a number of muscle fiber within a muscle. The motor neuron along with all the muscle fiber it stimulates is known as a motor unit. The motor end plate or the neuromuscular joint is the junction where the motor neuron connects with the muscle fiber it stimulates. When the action potential reaches the muscle fiber, each sarcomeres is stimulated causing cross bridging between actin and myosin filaments. The heads of the myosin fibers will attach to actin filament and pull it in center of the sarcomeres. This causes the sarcomeres to shorten.
The fourteen steps of muscle contraction are as follows:
1) Summation of EPSPs on -motor neuron causing membrane potential to reach a threshold potential: Excitatory postsynaptic potential (EPSP) are generated at the postsynaptic membrane. These EPSPs will summate on alpha motor neuron. Summation occurs when the postsynaptic membrane receives numerous impulses from many presynaptic terminals simultaneously or at short intervals that leads to addition of the impulses, causing the membrane potential to reach beyond its threshold levels. As a result, an action potential is generated at axon termini.
2) The action potential causes the voltage gated calcium channel to open. Calcium ions will enter the neuron and bind to proteins on the synaptic vesicle in the axon terminal of motor neurons.
3) The neurotransmitter acetylcholine is released in synaptic cleft
4) Acetylcholine (ACh) diffuses across the synaptic cleft and binds to its nicotinic acetylcholine receptors present in the sarcolemma, the cell membrane of the muscle fiber.
5) Binding of ACh to its receptors causes a conformational change in membrane channel for sodium ions. Sodium ions will enter and cause membrane depolarization. If depolarization reaches beyond the threshold, an action potential is reached
6) Action potential will travel along the sarcolemma and enter the muscle fiber via the T-tubules or transverse tubules
7) Voltage gated calcium channels will open. As a result, calcium is now released from its stores in the sarcoplasmic reticulum.
8) Tropomyosin blocks the myosin binding sites on actin. Calcium will bind to troponin, causing a shift in the position of tropomyosin. Tropomyosin will now moves away from the myosin-binding sites on actin. Hence, myosin binding sites will be unblocked.
9) Muscle contraction will now be initiated.
10) There is hydrolysis of ATP, releasing energy. ATP bound to myosin is hydrolyzed. ADP and Pi will remain bound to myosin S1 head.
11) Myosin head will now reach forward, and bind actin.
12) The actin filaments are shifted to center of the sarcomeres causing shortening of the sarcomeres. This is known as cross bridging. Contraction of myosin S1 segment is called power stroke.
13) Flexing of the myosin heads exposes the ATP binding sites. ATP will now bind to myosin.
14) Myosin will now detach from the actin binding sites. Energy from ATP causes myosin to return to its cocked forward position. The sarcomere is lengthened causing relaxation. Contraction and relaxation cycles are repeated.